1. Field of the Invention
This invention relates to the estimation, derivation and use of a signal-to noise-ratio (SNR)-related parameter, such as ES/N0, for symbols received over a communication channel.
2. Related Art
In wireless or wireline communications systems, it is often necessary or desirable to estimate a signal-to-noise ratio-related parameter for symbols received over a communications channel. One such parameter is ES/N0, where ES is the received energy/symbol, and N0 is the noise power spectral density. Another is EB/N0, where EB is the energy/bit, and N0 is again the noise power spectral density. The parameter EB/N0 is related to ES/N0 as follows:
            E      B              N      0        =                    E        S                    N        0              ⁢          R      bitspersymb      where EB and N0 are as defined previously, and Rbitspersymb is the number of source bits (pre-error correction coding) delivered by each channel symbol.
Both EB/N0 and ES/N0 bear a relationship with SNR and thus are properly characterized as SNR related parameters. For example, the parameter EB/N0 bears the following relationship to SNR:
            E      B              N      0        =            S      N        ⁢          (              W                  R          B                    )      where EB and N0 are as defined previously, S is the average signal power, N (=N0W) is the average noise power, S/N is the signal to noise ratio (SNR), W is the system bandwidth, and RB is the bit transmission rate.
Moreover, the variance, σ2, of the additive noise borne by the received symbols is related to the noise spectral density by the relationship
      σ    2    =                    N        0            2        .  
Before the symbols are input to a trellis decoder, for example, it may be necessary to estimate an SNR-related parameter such as ES/N0. The reason is that the computation of branch and state metrics performed during the decoding process may may be weighted and/or quantized differently depending on the relative value of the noise energy associated with the symbols being decoded. What's more, these signal-to-noise ratio estimates may be used to assist a user in pointing an antenna to ascertain which pointing direction leads to most effective reception.
For additional information on trellis decoders, including maximum a posteriori (MAP) decoders, log-MAP decoders, Max-Log-Map decoders, Viterbi decoders, and Soft Output Viterbi (SOVA) decoders, please see A, Viterbi, “An Intuitive Justification and a Simplified Implementation of the MAP Decoder for Convolutional Codes,” IEEE Journal On Selected Areas In Communications, Vol. 16, No. 2, February 1998, pp. 260-264; S. Benedetto et al., “A Soft-Input Soft-Output Maximum A Posteriori (MAP) Module to Decode Parallel and Serial Concatenated Codes,” TDA Progress Report 42-127, Nov. 15, 1996, pp. 1-20; D. Divsalar et al., “Turbo Trellis Coded Modulation with Iterative Decoding for Mobile Satellite Communications,” Proc. Int. Mobile Satellite Conf., June 1997; “A Comparison of Optimal and Sub-Optimal MAP Decoding Algorithms Operating in the Log Domain,” Proc. IC '95, Seattle, Wash. 1995, pp. 1009-1013; C. Berrou et al., “Near Shannon Limit Error-Correcting Coding And Decoding: Turbo-Codes,” Proc. IEEE Int. Conf. Commun., Geneva, Switzerland, 1993, pp. 1064-1070; L. R. Bahl et al., “Optimal Decoding of Linear Codes For Minimizing Symbol Error Rate,” IEEE Trans. Inform. Theory, Vol. IT-20, pp. 284-287, 1974, each of which is incorporated by reference herein as through set forth in full.
One approach for estimating total power involves analyzing Automatic Gain Control (AGC) settings at the receiver. However, this approach is cumbersome because an estimate of total received power does not easily translate into an estimate of an signal-to-noise ratio related parameter such as ES/N0. For one, it may also be impractical since extremely precise calibration of the AGC and other receive chain gain elements over voltage and temperature variations may not be easy. Moreover, knowledge of the noise figure of these same elements may not be characterized, especially over temperature variations. Another approach for estimating total power is to estimate the noise power, Np, by (1) computing the sample variance of √{square root over (I2+Q2)}, (2) estimating the ‘signal power+noise power’, Sp+Np by computing the mean of I2+Q2, and forming the appropriate ratios and subtractions to derive the signal to noise ratio Sp/Np, and then (3) converting Sp/Np to the ES/N0 using the relations Sp=ES and N0/2=Np. Here I and Q are the in-phase (I) and quadrature phase (Q) components of quadrature symbols received at the receiver. However, this computational process is involved.